1. Field of the Invention
The invention relates to engineered particles useful for interleaf toughening of composite articles. More specifically, the invention relates to engineered cross-linked particles having a thermoplastic polymer backbone, which particles are insoluble in resin systems and remain discrete particles after curing.
2. Description of the Related Art
Functionalized acrylonitrile-butadiene rubbers have been used for many years to toughen thermoset adhesives and composites; U.S. Pat. Nos. 3,926,904 and 4,500,660. These tougheners have been shown to be soluble inside the uncured thermoset resin, but then phase separate out during cure, generating regions of rubber throughout the body of the matrix.
These rubber particles are also claimed to be cross-linked, possibly by “in-situ” formation by RIPS (resin-induced phase separation) as commonly described. These rubbers were proven effective in regards to toughening, however they generally decreased the hot wet mechanical performance of the composite. This drop in hot wet performance limited rubber's utility in aerospace applications.
Functionalized and non-functionalized thermoplastics, such as polyethersulfones, were also found to increase toughness in composites but without significant loss of hot wet performance; U.S. Pat. No. 4,656,207. These thermoplastics behaved similar to the above mentioned rubbers; dissolving into the uncured resin but phase separating from the resin during cure.
Hirschbuehler et al. U.S. Pat. Nos. 4,539,253; and 4,604,319 showed that by concentrating the toughener between the plies of a composite, a greater increase in toughness could be obtained. This concept was utilized through the insertion of thermoplastic particles, into the resin, that would remain mainly insoluble during prepreg manufacture, but would later dissolve into the resin and then phase separate during cure, U.S. Pat. Nos. 4,954,195; 4957,801; 5,276,106; and 5,434,224. These particles were large enough that they were filtered by the fibers to the interleaf region between the plies. Thus, when particle dissolution occurred, a higher concentration of thermoplastic could be generated in the interleaf region than would be possible through the dissolution of the thermoplastic before prepreg manufacture.
Another approach taken to increase the toughness in the interleaf region was through the insertion of insoluble particles. Numerous patents have been filed by Gawin and others describing the insertion of pre-formed rubber particles; U.S. Pat. Nos. 4,783,506; 4,977,215; 4,977,218; 4,999,238; 5,089,560; and 6,013,730. These particles were again large enough so that they would be filtered away from the fiber bundles into the interleaf region. Also, though they were insoluble they were capable of swelling in the resin. Later technology, U.S. Pat. Nos. 5,266,610; and 6,063,839, used core-shell rubber particles to be used for the same purpose. Likewise, silicone based particles were also developed; U.S. Pat. No. 5,082,891, for toughening purposes.
Insoluble thermoplastic particles were utilized as interleaf tougheners to avoid the loss of hot wet performance as indicated by U.S. Pat. Nos. 4,957,801; 5,087,657; 5,242,748; 5,434,226; 5,605,745; and 6,117,551. However, these insoluble particles are generally made from polymers that do not dissolve or swell in the resin compositions, and are made from precipitation or milling.
Several variations of polyamide/Nylon particles have been explored to enhance the particle's toughening ability. One variation, U.S. Pat. No. 5,028,478, examined the insertion of a crosslinked epoxy-amine network into the particle to increase the particle's solvent resistance. The others, U.S. Pat. Nos. 5,169,710; and 5,268,223, examined the generation of a porous particle that could increase the interaction between the particle and the resin.
Many high Tg thermoplastics, such as nylon or polyetherimide sold under the trademark ULTEM®, are so tough that they cannot be milled/grinded without cryogenic conditions. To effectively make use of these polymers, and polymers of similar kind such as the PPO/PS alloys, a method needs to be established to produce the particles in the usable particle size.
Accordingly, the thermoplastic particles for composite toughening and methods presently available for producing such particles require further improvement. Thermoplastic particles remaining insoluble even after curing, thereby imparting improved toughness, damage tolerance, hot wet performance, processing, micro-cracking resistance, and reduced solvent sensitivity would be a useful advance in the art and could find rapid acceptance in the large commercial transport and/or military aerospace industries, among others.